The present invention relates to an optical source. More specifically, the present invention is concerned with an optical source which can be configured to emit a regular stream of single photons or a regular stream of pulses of n photons (where n is an integer).
With the emergent fields of quantum communications, cryptography, teleportation and quantum computation, there is a need for a source which can be controlled to produce single photons or pulses of a fixed number of photons on demand. A single photon source is a particularly secure source because any unauthorised attempt to read information from this source can be detected. Also, a single photon source provides a source of strongly sub-Poissonian light which has a very high signal to noise ratio.
Recently, there have been attempts to make a single photon source based on an electron-turnstile device. This work has been reported by Kim et al, Nature, 397, 500 (1999). This device comprises a quantum dot. An AC voltage is applied to the device to drive an electron into the dot during the first half of the voltage cycle, then a hole is driven into the dot in the second half of the cycle. The hole and the electron will combine and a photon is emitted. The regular spacing of photon detection events arises due to regular pumping of electrons and holes into the quantum dot via an AC voltage. However, this device shows poor electron current quantization and, as a result, the photon xe2x80x9ccurrentxe2x80x9d is noisy. The device can only operate with maximum frequencies in the MHz range which results in a low photon emission rate. Also, the device is only capable of operation at a temperature of around 50 mK.
Single photon states may also be produced using down-conversion or with a highly attenuated laser. The latter method is currently used in secure quantum communication channels. Here, the laser is set up to produce less than 1 photon per pulse such that only some pulses actually contain a photon, other contain none. However, this method is unreliable as, due to the Poissonian nature of laser light, some pulses contain more than one photon.
It has also been suggested to use a surface acoustic wave (SAW) to pump a dot which can trap a single electron and a hole (Wiele et al. Phys Rev. A, 58, R2680 (1998)). A SAW propagating on a piezo electric material is accompanied by a travelling wave of electrostatic potential. Free charge can interact with the electrostatic potential, and an acoustoelectric current can be created. If the potential is sufficiently strong, it can be used to spatially separate photo-excited electron-hole pairs where the electrons are held in the minima of the electrostatic wave, and the holes are held in the maxima Recombination is therefore suppressed, and the charges are carried along by the electrostatic potential. In the device described in the paper, a SAW is used to spatially separate photo-excited electrons and holes into alternate xe2x80x9cwiresxe2x80x9d of electrons and holes which move with the SAW.
A xe2x80x9cstressorxe2x80x9d dot is provided which is a quantum dot formed by local potential minimum in a buried quantum well caused by a structure on the surface of the device. The stressor dot attempts to trap an electron and a hole from the moving xe2x80x9cwiresxe2x80x9d. The electron and hole then recombine in the stressor dot. This device suffers from the problem that a dot with the postulated properties necessary for operation has never been fabricated, and it is not clear how it would be fabricated. It is also possible that the electrons and holes may just be swept past the dot and not be trapped.
The present invention addresses the above problems, and in a first aspect provides a photon source, comprising a first semiconductor region having excess carriers with a first conductivity type, and a second semiconductor region having excess carriers with a second conductivity type opposite to that of the first conductivity type, means for creating a surface acoustic wave (SAW) travelling from the first semiconductor region to the second semiconductor region such that excess carriers from the first semiconductor region are carried by the wave to the second region and quantizing means for quantizing the carrier transport caused by the wave such that the number of carriers introduced in the second semiconductor region can be controlled to the accuracy of a single carrier.
The source can be configured such that an integer number of carriers are introduced into the second semiconductor at regular intervals. For a single photon source, the source is configured such that single carriers one at a time are introduced into the second semiconductor region at regular intervals.
The carriers of the first conductivity type can either be electrons or holes. For the purposes of this explanation, the device will be discussed only with electrons as the first carriers with the first conductivity type. However, it will be appreciated by those skilled in the art that the device could be operated with holes as the carrier with the first conductivity type.
The applicants do not wish to be bound by any specific theory as to the operation of the device. However, it is believed that the surface acoustic wave (SAW) is accompanied by a travelling wave of electrostatic potential which modulates the conduction and valence bands. The first region and the second region of the device form a P-N junction. (There may be an insulator between the p and n regions).
Electrons are carried from the n-type (first) region into the p-type (second) region by the SAW potential. The SAW forces the electrons to be carried in the minima of the electro-static potential. When electrons arrive at the p-type region, they recombine with the holes to produce photons.
The source is provided with quantizing means which quantizes the transport of the carriers carried by the SAW potential such that the number of electrons located in each SAW minimum can be controlled to the accuracy of a single carrier. The quantizing means are located such that they quantize the transport of the carriers before they are introduced into the second region. The quantizing means can control the number of carriers in the SAW minima to be 1, 2 or more if required
For the purposes of this explanation, it will be assumed that a single electron is located in each SAW minimum. However, quantizing means could be controlled such that more than one electron is located in each SAW minimum. Therefore, after the SAW has travelled through the quantizing means, a single electron is introduced into the p-type region per cycle of the SAW field. The electron can then recombine with an excess hole.
In the source of the present invention, a stream of single photons or pulses of n-photons are produced by introducing or injecting carriers (an xe2x80x9cinjection eventxe2x80x9d) of one type into an environment where they can combine with carriers of the opposite type to emit a photon (a xe2x80x9cphoton emission eventxe2x80x9d). To produce an ideal single photon source, there is a need for a strong correlation between the injection events and photon emission events.
A temporal uncertainty arises in the photon emission events as the carriers may not recombine immediately to emit a photon. In addition, missing events occur as the carriers may non-radiatively recombine which does not result in the emission of a photon. To achieve a strong correlation between injection events and photon emission events, the uncertainty in the time of the photon emission events must be smaller than the time between injection events. Preferably, the uncertainty is at most 50%, more preferably at most 10%, even more preferably at most 1% of the time between injection events. The requirements on uncertainty are application-dependent.
The total recombination time primarily dictates the temporal uncertainty in the photon emission events. As the electrons are introduced into the p-type region with the SAW, the SAW period is, in this case, the time difference between injection events. Generally, the radiative recombination time is greater than or equal to the SAW period, (for example, in GaAs, the radiative recombination time will be several nanoseconds and the SAW period will be typically tenths of nanoseconds). Hence, the electrons will accumulate in the p-type region. This results in the loss of correlation between injection events and photon emission events, as several photons could be emitted in a SAW cycle which injects only one electron into the p-type region
The above preferred criteria also apply to an n-photon source where each cycle of the field will inject n electrons into the p-type region with the aim of producing n-photons from the p-type region for every cycle of the SAW field. The following description, will relate to a single photon source. However, it will be appreciated that the source could also be an n-photon source where n electrons are injected into the p-type region during each injection event.
The source can be configured to prevent accumulation of electrons from different injection events in the p-type region. This can be achieved in a number of different ways. For example, the source could be provided with means to control the rate of introduction of carriers of the first type into the second region, i.e. increase the time between electron injection events.
The time between electron injection events can be increased by providing a selective transmission means which is configured to transmit electrons for every m cycles of the SAW, where m is an integer greater than 1. This results in the time between electron injection events being m times the SAW period. Thus, this fulfils the requirement since m times the SAW period can be made much greater than the total recombination time.
The selective transmission means can be provided by a pulse bias means which provides a pulse bias to the quantizing means such that the quantizing means can only transmit carriers every m cycles of the SAW.
The selective transmission means could also be provided by a current splitting device, the current splitting device comprising an input, first and second outputs and control means for selecting the required output, wherein carriers emitted from the first output are injected into the second region and carriers emitted from the second output are not injected into the second region, the control means being configured to emit carriers from the first output every m cycles of the SAW. Preferably, the second output injects electrons into an n-type region which is connected to the first n-type region.
Instead of or in addition to increasing the time between injection events as described above, the second region could be configured to decrease the uncertainty in the time of the photon emission event. An upper limit can be defined for the uncertainty in the photon emission time by limiting the length of the second region in the direction of the propagation of the SAW. The length of the second region could be limited so that the time which electrons take to traverse the second region is much less than that of the SAW period. For example, the time for traversing the second region could be at most half, more preferably a tenth, even more preferably 100th of the SAW period.
If electrons are not introduced into the second region with every SAW period, (for example, if they are only being introduced every xe2x80x98mxe2x80x99 periods by a selective transmission means as described above,) the length of the second region could be increased by a factor of m.
A third region may also be provided on the opposite side of the second region to the quantization means. The third region will not have an excess of the carriers of the second type.
In addition to or instead of any of the above methods for increasing the time between injection events and decreasing the uncertainty in the time of photon emission event, the temporal uncertainty in the time of the photon emission events could also be decreased by providing means to enhance the non-radiative recombination rate. An electron which has just entered the p-type region can recombine either radiatively (which results in the emission of a photon) or non-radiatively (no emission). The total recombination time fixes an upper limit on the uncertainty in photon emission time, because any electrons which do not combine radiatively will be removed by non-radiative recombination.
It should be noted that if the non-radiative recombination rate is much smaller than the total radiative recombination rate, then the non-radiative recombination rate dictates the total recombination rate.
Such means could be impurities provided to the second region. Such impurities could be added during growth or implanted afterwards. Suitable impurities will occupy a deep level in the semiconductor band gap for example in a GaAs based semiconductor, Cr and/or Fe could be used. Also, a shallow donor such as Si could serve the same function if the temperature of the device is low enough so that an electron trapped by the shallow donor would not escape.
Such impurities may be provided in a concentration between 1015 to 1018 cmxe2x88x923.
The non-radiative recombination rate may also be increased by growing the second region at a low temperature, for example less than 450xc2x0 C.
The device could also be configured to comprise a microcavity. To comprise a microcavity, the source is provided with reflective surfaces or layers on opposite sides of the second region. The microcavity increases the radiative recombination rate. This phenomenon is described in Bjork et al, Phys Rev. A, 44, 669 (1991). The reflective surfaces could be achieved by forming Bragg stacks on either side of the second region.
A metallic gate which can be formed on one or both sides will also serve as a reflective layer.
Preferably, the excess carriers in the first region will be provided in a two dimensional carrier gas. Similarly, it is also preferable if the carriers in the second region are also provided in a two dimensional carrier gas. However, the carriers in the first and/or second region could be contained in a bulk 3D region.
The source may be configured such that the first and second regions are located within the same semiconductor layer. Alternatively, the first and second regions may be located in different layers. For example, the first region may be a high mobility layer, and the SAW may carry excess charge through the quantizing means, when the charge is still in the high mobility layer. The second region may then be a low mobility layer and the excess carriers carried by the SAW are transported into this low mobility layer for recombination. For example, the excess charge could tunnel into the second region for recombination.
The first and second regions may also be located in the same layer, but at different interfaces of that layer. The different interfaces could have different mobilities, hence, the first region could be located towards the high mobility interface and the second region could be located towards the low mobility interface. For example, the different interfaces could be the first grown (or lower) interface and the second grown (or upper) interface. The charge could move from one interface to the other as required.
Forming the first and second regions in different layers or at different interfaces of the same layer allows a device to be fabricated where the second region is grown at a lower temperature and/or with impurities in order to enhance the non-radiative recombination rate.
For the avoidance of doubt and as used hereinafter, a source where the carriers of the first conductivity type can be switched between different interfaces or different layers will be referred to as a source where the carriers of the first conductivity type can be switched between different channels.
To operate the device, Ohmic contacts are provided to the first, second and third (if present) in order to complete a circuit within the source and prevent charge accumulation.
The quantizing means will preferably provide a one dimensional or quasi one dimensional restriction for the carriers coming from the first region. Normally, this will be provided by a quantum point contact or a split gate. The quantizing means will have an opening. Preferably, this opening is configured to optimise quantization of electron transport This can be achieved if the opening of the quantizing means which is provided towards the leading edge of the SAW will be smoothly varying. For example, the opening may be rounded A study of the design of such quantizing means is discussed in Talyanksii et al, Phys. Rev. B, 56, 15180, 1997-II.
The quantizing means may also comprise a plurality of quasi one dimensional restrictions located across the path of the SAW. In this case, there will be a plurality of channels carrying excess carriers for recombination.
As has been previously mentioned, the SAW provides a wave in the conduction band over the n-p junction. In order that the SAW carries the electrons over the n-p junction, it is preferable if the transition from the n to the p-type region is smoothly varying in energy, so that electrons continue to be confined by the SAW potential.
This may be achieved by providing an insulating region between the first and second regions. The insulating region may be an undoped semiconductor. Alternatively, the insulating region may be induced by providing a gate which is capable of inducing an insulating region between the first and second regions.
Typically, the length of the insulating region in the direction of propagation of the SAW will be 1 mm or less.
It is also possible to smooth the transition in energy from the first to the second region by applying a potential to the first and second regions such that the first region is biased with respect to the second region. Thus, the height of the potential barrier between the first and second regions can be lowered. (It should be noted that if the height of the potential barrier is lowered too much then electrons could flow from the first region to the second region without the SAW. Obviously, this should be avoided). To reduce the height of the barrier, it is preferable if the source is provided with Ohmic contacts to the first and second regions, such that the first and second regions can be biased with respect to one another. These Ohmic contacts may or may not be the same as the Ohmic contacts used to complete the circuit.
In order for the device to work, a SAW must be created Piezoelectric materials will result in a wave of electrostatic potential accompanying the propagation of a SAW. Any structure which does not have inversion symmetry will demonstrate piezoelectric properties in at least one crystallographic direction. Most III-V materials and II-VI materials are piezoelectric. For example, GaAs, InAs, AlAs, GaSb or alloys thereof for example AlGaAs, InGaAs etc.
As piezoelectric properties are usually orientation dependent, it is necessary to orient the SAW transducer such that the SAW propagates in a direction where the material exhibits piezoelectric properties. Preferably, the SAW transducer is oriented such that the SAW propagate in a direction where the material exhibits the optimum piezoelectric properties.
The SAW will generally be produced using a SAW transducer which comprises a plurality of interdigitated contacts wherein each finger of the transducer is separated by a half SAW wavelength. The fingers will produce a SAW propagating in a direction perpendicular to the length of the fingers.
The SAW serves to spatially separate electrons and holes. In order for recombination to occur, electrons and holes must not be spatially separated by the SAW potential in the second region. A moderate hole density in the second region should suffice to completely screen the SAW potential. If the SAW potential is screened then electrons will be released allowing recombination between electrons and holes. To ensure that the SAW potential is screened in the second region, it is preferable if a surface gate is provided overlying the second region.
The present invention requires the presence of excess electrons and excess holes which are laterally arranged such that a SAW can carry electrons into the region with excess holes. There are many ways of providing the excess carriers.
The first and second regions will generally be formed in an active region. The active region may be a plurality of layers or a single layer, for example a layer capable of supporting a two dimensional carrier gas. The excess carriers may be provided in this active region by a doped layer which is provided either adjacent or closely spaced to the active region. For example, the doped barrier layer could be a modulation doped barrier layer comprising an undoped spacer layer provided between the active region and the doped barrier layer. The doped barrier layer may be provided above or below the active region. Alternatively, or in addition to a barrier layer, the carriers may be induced by a gate provided above or below the active region having the carriers, i.e. a front or back-gate.
For the avoidance of doubt as used hereinafter, the term xe2x80x9cbarrierxe2x80x9d layer will refer to a layer with a larger band gap than that of at least one of the layers in the active region. More preferably the barrier layer will have a larger band gap than all of the layers in the active region. A reference to a first layer overlaying a second layer, means that the first layer is situated on an opposing side of the second layer to a base region. The first and second layers do not have to be in contact. A base region can be a semiconductor substrate, a semiconductor substrate and overlaying layers or any layer on which the source is formed.
The source preferably has first and second carrier providing means to induce or supply carriers into the first and second regions. For example, either or both of the first and second carrier providing means can be a gate, a doped barrier layer, or a combination of the two.
Where gates are provided, the gate which is used to induce carriers on the first region will be referred to as a first gate and the gate for the second region will be referred to as a second gate. Either or both of the first and second gates can be a front gate. Similarly, either or both of the first or second gates can be back gates.
The SAW will generally travel on the surface of the source. Therefore, there needs to be a relatively smooth (on the SAW wavelength scale) transition of the source structure along the surface of the source. Conventionally, the source will be formed such that the surface along which the SAW propagates is substantially perpendicular to the growth direction of the device. Thus, the first and second regions of the device are often laterally arranged with respect to the growth direction. Current fabrication techniques generally involve the layer by layer growth of a structure, hence, it is easy to produce a device with variations in its layers in the growth direction. However, producing a variation in the layers perpendicular to the growth directions, i.e. a lateral variation, is less straightforward.
In a second aspect, the present invention provides a method of forming a photon source, the method comprising the steps of:
a) growing an active region overlying a semiconductor base, the active layer having a first conductivity region and a second conductivity region, the first conductivity region having an excess of carriers of a first conductivity type and the second conductivity region having an excess of carriers of a second conductivity type, the first and second regions being laterally arranged with respect to the growth direction;
b) forming wave generating means for providing a surface acoustic wave (SAW) which propagates in a direction such that it travels from the first region into the second region;
c) forming quantizing means between the first and second regions to quantize the carrier transport caused by the SAW.
The active region may be a single layer such as a quantum well layer or a plurality of layers. The semiconductor base may be a semiconductor substrate or it may be plurality of layers which may or may not comprise a semiconductor substrate.
It has been previously mentioned that the carriers to the active region may be supplied by a doped barrier layer. Therefore, the method preferably further comprises the step of forming a doped barrier layer either above or below the active region. The same barrier layer does not have to supply the carriers of both the first and second conductivity type However, preferably the step of forming the barrier layer comprises the step of forming the barrier layer with a variation in its doping in a direction lateral to that of the growth direction.
Fabricating such a device with a lateral variation in the doping of a barrier layer will generally require the use of special fabrication techniques. Many fabrication techniques can be used such as regrowth and ion beam techniques (for example ion beam implantation techniques and ion beam damage techniques).
The device can be fabricated by a regrowth technique by etching the semiconductor base to form two growth planes, for example, a facet could be exposed located between upper and lower parallel planes. If the substrate plane is (100), the structure could be etched to form two 100 planes which are separated by say an (n11)A plane, where n would typically be 4 or less. Any planes which are approximately (n11)A with a suitable n may also amphoterically dope p-type.
If such a structure is formed in GaAs using a silicon dopant, (silicon is an amphoteric dopant), the silicon will n-dope any overgrown doped layers on the (100) planes but will p-dope the doped layers found on the (n11)A planes. A person skilled in the art could easily determine the appropriate growth conditions. Typically, growth at about 590xc2x0 C. to 630xc2x0 C. with a low As4 pressure will produce the correct doping. Therefore, it is possible to obtain a structure comprising a P-N junction. The substrate could also be a (n11)A plane and a (100) ridge could be etched onto the substrate such that the ridge or facet dopes n-type whereas the planes parallel to the substrate surface dope p-type.
Thus, it is possible to produce a barrier layer with a lateral variation in its doping such that both holes and electrons can be induced in the quantum well layer. The barrier layer can be formed either above or below the quantum well layer.
Alternatively, the variation in the doping of the barrier layer could be achieved by implanting the barrier layer with p and n type dopants during growth to produce the required doping variation. The n and or p type dopants could be implanted using focused ion molecular beam epitaxy (FIMBE).
The excess carriers in the first region and the second region can be induced by first and second gates respectively. These gates may both be supplied on the semiconductor base side of the active region (i.e. below the active region) or they may be supplied on the opposing side of the active region to the semiconductor base i.e. above the active region. Alternatively, one gate may be supplied above the active region and the other may be supplied below the active region.
The first and second gates may also be provided in the same layer as it is also possible to produce a back or a front-gate with a lateral variation in its doping. Therefore, the device preferably comprises a highly doped gate layer which is doped with an amphoteric dopant, the layer being formed on at least two planes, wherein at least one of the planes causes n-type doping and the other plane causes p-type doping. Highly doped means that the transport characteristics of the layer are generally metallic.
To produce this type of structure, the method of fabricating the device comprises the steps of etching the semiconductor base to form at least two different growth planes and growing a heavily doped gate layer which is doped with an amphoteric dopant overlying the at least two growth planes.
The gates are used to induce carriers of the appropriate conductivity type in the first and second regions. The gates can be used either instead of or in addition to a barrier layer which has a lateral variation in its doping. The gates can be of the same conductivity type or opposing conductivity types. It is also possible to use gates of the same conductivity type where the barrier layer does not have a variation in its doping across the first and second regions. A single gate could also be used and the source could be biased with respect to this single gate such that carriers of opposing conductivity types are provided to the first and second regions respectively.
The above devices have been formed overlying a facet where the facet has been etched through a single material and just serves to vary the orientation of the growth plane to allow for amphoteric doping. However, further advantageous variations on the device are possible if the facet intersects two or more layers. Varying the composition of the layers which the facet intersects allows the confinement potential of carriers on the facet to be modified.
Further, one or more of the layers intersected by the facet may be a gate layer i.e. a heavily doped semiconductor layer. This gate may be formed at either end of the facet i.e. close to the junction of the facet with the upper or lower plane. The gate can be biased to deplete out carriers to form an insulating region between the first region and second region.
Also, a gate may be provided towards the centre of the facet in order to induce carriers of the second conductivity type on the actual facet. Either of the xe2x80x9cfacet gatexe2x80x9d layers could be a p-type layer or an n-type layer regardless of the conductivity type of the second conductivity type.
As there is no real need to provide a doped barrier layer with a lateral variation in its doping, providing that gates or some other means are provided to induce excess carriers of different conductivity types in the first and second regions, it is also possible to use a re-growth technique to fabricate the structure which does not require the use of amphoteric dopants or require an etched semiconductor base.
A substrate or a substrate with buffer layer(s) is overgrown with a heavily doped layer. The n-type layer is then etched to form an island The etch is preferably a selectively plane etch which produces smooth (on the scale of SAW wavelength xcex) side walls at the edges of the island. A heavily doped p-type layer is then overgrown over the substrate/buffer layer and n-type island. This p-type layer is then etched to form a p-type island which is spatially separated from the n-type island. The p and n-type islands can be used as back gates for the structure. The active layer and barrier layers can then be overgrown over the p and n-type gates. Of course, it will be appreciated that either the n-type gate or the p-type gate could be formed first
It should also be noted that it is not necessary for the gate which is used to induce the holes and the gate which is used to induce the electrons to be of opposing conductivity types. It is possible to induce holes using an n-type gate and it is also possible to induce electrons using a p-type gate. Therefore, a doped p or n-type semiconductor layer can be growth over a planar substrate or buffer layer to form the back gate. This doped layer can then be etched to form two isolated islands which will be the two back gates. These islands are preferably formed with sloping side walls to allow for smooth overgrowth of the remaining layers.
Also, as previously mentioned, the first and second gates could be formed on opposing sides of the active region. Either the first or the second gate could be formed by any of the above techniques and a surface gate could be used for the remaining gate.
It is also possible to use FIMBE to form a gate layer which is embedded into the structure which has both p-type and n-type laterally arranged gates. The gates produced could be a p-type and an n-type gate. Alternatively, the technique could be used to dope two spatially separated regions with the same conductivity type (i.e. either both n-type or p-type). It is also possible just to dope a single region to form a single back-gate.
It is also possible to use a technique which relies on ion beam damage. It has been found that if a thin layer of heavily doped semiconductor (for example a layer less than 200 nm) is implanted with high energy ions, then these ions destroy the conductivity of the layer. Therefore, it is possible to pattern a layer into conducting and non conducting sections. This can be used to fabricate one or two spatially separated islands to form one or two back-gates as required.
The above techniques can be used in isolation or in combination with each other. For example, both ion beam fabrication methods and amphoteric doping techniques can be used to fabricate a single device.
It should also be noted that a front gate could be used on either or both of the first or second conductivity regions. However, it may be less desirable to put a front gate on the first conductivity region as this could cause screening of the SAW.
It is also desirable to be able to collect the light emitted from the second region with minimum loss. Therefore, it is preferable if the structure is provided with a wave guide layer. Such a wave guide layer is preferably provided over or under at least part of the second semiconductor region. For example, the second region could be embedded in a waveguide structure. Another way of achieving directed photon emission (i.e. photons preferentially emitted in a small solid angle) is to use a microcavity structure. Therefore, it is also preferable if the second region has either a reflecting layer above the active layer, or a reflecting layer below, or both.
Typically, SAW wavelengths are between 0.25 xcexcm to 10 xcexcm, but will usually be about 1 xcexcm or less for this device.
In a third aspect, the present invention provides a method of generating photons, the method comprising the steps of creating a surface acoustic wave (SAW) which travels from a first region having an excess of carriers with a first conductivity type to a second region having excess carriers with a second conductivity type, the first and second conductivity types being opposing conductivity types, such that excess carriers from the first region are carried by the wave to the second region; and quantizing the carrier transport caused by the wave, such that the number of carriers introduced into the second region can be controlled to the accuracy of a single carrier.